1. Field of the Invention
This invention relates to liquid adhesive sealant compositions which are shelf stable in the presence of oxygen but which rapidly cure at room temperature with low shrinkage when exposed to anaerobic conditions as on confinement between closely facing, gas impervious surfaces.
2. Description of the Prior Art
One component shelf stable structural adhesive and sealant systems are of considerable commercial interest because they offer greater user convenience than the conventionally employed two component structural adhesives and sealants, especially if they can be cured at ambient temperatures. In the past, a considerable amount of research and development has been carried out to develop such products. Thus, one component epoxy compositions have been developed as adhesives, but they require extended elevated temperature cures to attain useful strength properties. Urethane and silicone based adhesives and sealants have become available which cure at ambient temperatures but their cure mechanism is dependent on the availability of water, usually from the atmosphere, which results in serious limitations in their efficacy for many applications. Also cyanoacrylate types of adhesives have been introduced into the adhesives markets which cure in a matter of seconds at ambient temperatures, again by reaction with moisture, even though they have very definite processing and application limitations particularly as regards types of substrates, durability and chemical resistance and are supplied in a form which can cause severe injury to the skin and eyes.
Perhaps the most successful one-component ambient temperature curing adhesive systems which are now employed, are so-called anaerobic compositions which comprise catalyzed polymerizable mixtures of various monomeric acrylate ester monomers. They are normally liquids remaining in that state as long as they contain an adequate amount of oxygen, such as atmospheric oxygen, and polymerization or cure will not commence even though they may contain substantial amounts of room temperature polymerization initiators. When such anaerobic compositions are however placed between surfaces that are relatively impervious to oxygen, the stabilizing effects of atmospheric oxygen is removed and cure commences within a relatively short time even at ambient temperatures.
The anaerobic adhesive products, now furnished commercially, would find many more applications if certain of their critical performance parameters could be improved. These include enhanced strength properties, particularly toughness, since the cured anaerobics tend to be rather brittle; resistance to a wider range of chemicals; a greater useful service temperature range, which is at present quite limited both as regards to upper and lower service temperatures; and extended range of applicability to different substrates which are now restricted mainly to specific metal surfaces such as brass, steel and aluminum. Also, cure speeds vary with different metals ranging from fast with brass to slow for zinc and for certain finishes on metals such as anodizing, plating and passivating, which will slow or prevent cures. Special primers may have to be used to speed cures. If required, they obviously lessen the all important user convenience factor of an ambient temperature curing one-component adhesive system. An additional vexing problem with the currently produced anaerobic adhesive compositions is that they are most effective only for bonding surfaces with close clearances and lack gap holding characteristics. This is due to a considerable extent to their substantial shrinkage upon cure. Larger clearances furthermore require more material which significantly reduces cure speed and, thus, lowers productivity in assembly bonding operations. Also, cure conditions are quite temperature sensitive, lower temperatures requiring much longer cure times. Cure at 40.degree. F., for example, may require weeks. After many years of research, these problems have not been satisfactorily resolved.
Patents considered to be representative of the state-of-the-art include U.S. Pat. No. 3,249,656, which describes an adhesive based upon an acrylic diester of an ethylene glycol; U.S. Pat. No. 3,046,262 directed to a particular class of unsaturated monomers for use in formulating anaerobic curing compositions; U.S. Pat. No. 3,454,543 which discloses particular amine compounds used as accelerators in anaerobic curing adhesive compositions and U.S. Pat. No. 3,544,536, which describes a group of oxyalkyl amine stabilizers for use in anaerobic sealant formulations.